Human bone marrow under the influence of human IL-3 and in suspension culture gives rise to 10 to 30% basophils. When human bone marrow is cultured on the surface of agarose in the presence of IL-3, both basophils and mast cells can be identified that reach maximum numbers by 2 to 3 weeks. The IL-3 dependent growth of human basophils and mouse mast cells in suspension culture is inhibited by granulocyte-macrophage colony stimulating factor. The IL-3 dependent growth of human basophils and mast cells in culture is not influenced by the removal of cells bearing T cell, B cell, or macrophage markers. Human lung mast cells release both heparin and chondroitin sulfate E. Mast cell proteoglycans are susceptible to cleavage by both hydroxyl radicals and hypochlorous acid. Bi-directional phenotypic changes in mast cells can be identified in vivo. This demonstrates that local factors also play an important role in the final stages of mast cell differentiation.